Michelle Williams does her best Marilyn Monroe for the October issue of Vogue, and the 31-year-old actress says preparing to portray the Hollywood icon "felt like being reborn." But first, she had to get over her fears.

"As soon as I finished the script, I knew that I wanted to do it, and then I spent six months trying to talk myself out of it," Williams tells Vogue. "But I always knew that I never really had a choice. I've started to believe that you get the piece of material that you were ready for."

The movie spotlights the turbulent behind-the-scenes story of "The Prince and the Showgirl's" filming in 1956, and revolves around Marilyn Monroe and set assistant Colin Clark (played by Eddie Redmayne).

Although Williams tried at first to gain weight to replicate Monroe's dangerous curves, she wound up settling for foam padding at the hips. After help from a choreographer to learn the walk and six months of intense study, the actress felt as though she'd stepped right in to Monroe's persona.

"It felt like breaking my body and remaking it in her image, learning how she walked and talked and held her head," she says. "None of that existed in my physical memory, and I knew I needed as much time as possible to make it part of me."

Off camera, though, Williams says she's gone through a transformation of her own.

"I feel like something has changed for me, but it's a new change, so it's going to be hard for me to describe," she tells Vogue. "Maybe it has something to do with turning 30. I don't feel as shy or nervous or self-conscious. I have more confidence that I can handle what life brings me. I don't feel scared to have an idea and express it. I feel giddy about it...It's like I found my voice."